A bill validator typically includes a bill path and a transport system for guiding the bill past a recognition sensor area and then to a stacking area where the bill is stored in some sort of cash box. Such validators typically include a system to prevent fraud. In one type of fraud, the thief uses a string connected to a bill to retrieve the bill after authentication and still receive a product or service. These xe2x80x9cstringsxe2x80x9d are mechanical attachments to the bill, which can be manipulated externally. Such strings may take many forms including wires, tapes, extruded materials and the like. This kind of fraud is typically known as a xe2x80x98string cheatxe2x80x99.
Various solutions have been used to solve the string cheat problem. For example, systems have been designed to thwart string cheating by detecting the presence of pull strings optically or mechanically, by preventing a shutter from closing, or by using some form of unidirectional or actively controlled mechanical arrest. Optical detection of strings has been challenged by the use of finer transparent strings.
Presented is a polarizer based detector for a currency validator. An embodiment is a string detector that includes a string fraud detection means arranged along a transport path of the validator, wherein polarized light is used to detect the string.
Implementations of the invention may include one or more of the following features. The string fraud detection means may include at least a light source and at least a photo detector, and the photo detector may be a polarized detector means. The light source may be a laser diode, and may be composed of at least an LED and a polarizer or may include two polarizers, which may be linear polarizers or circular polarizers. If circular polarizers are used, one polarizer may be right-handed and the second may be left-handed or the two polarizers have the same handedness. The axis of the two linear polarizers may be crossed at substantially 90xc2x0 and the axis of the polarizers may be oriented substantially at 45xc2x0 to the transport path. The polarizers may be active in a limited range of wavelengths, and may be active in the visible wavelength range and inactive in the IR wavelength range. The string fraud detection means may include at least a light source, a detector and at least one polarizer means on one side of a transport path and a mirror on the opposite side, such that polarized light is reflected towards the detector through the polarizer. The string fraud detection means may include a plurality of light sources and polarizing means, wherein at least one source has a wavelength in a range that is polarized and at least a second light source has a wavelength in a range that is not polarized. The transport path may include at least one transparent window, and the transparent window may be made of at least one of PMMA, cycloaliphatic acrylic, optical grade acrylic (PMMA), allyl diglycol carbonate, modified urethane and glass. An optical subassembly may form the transparent window, and the optical subassembly may include a frame molded around a rectangular glass insert, wherein the frame may be formed of a low shrink material. The optical subassembly may be loaded as an insert into an injection mold tool that forms a portion of the transport path, and grooves may be formed in a portion of the transport path near the location of the optical subassembly to absorb stress due to mold shrinkage. The transport path may include at least one window element and polarizer component. The polarizer based detector may include sensor means, validation means, comparison means and associated memory means.
Another aspect of the invention includes a method for detecting a transparent string in a currency validator. The technique includes illuminating the string with polarized light, and detecting the polarized light using at least a photo detector and at least a polarizer, wherein the polarization of the light is rotated through the string.
Implementations of the method may include one or more of the following features. The technique may include detecting the rotated light by transmission through a polarizer, or detecting the rotated light by absorption by a polarizer. Polarized light in a limited range of wavelengths may be used to detect a transparent string, and opaque strings may be detected with light in another range of wavelengths. Transparent string may be detected in the visible wavelength range and the opaque string may be detected in the IR wavelength range, and a signal may be measured to detect the presence of a string and/or the signal may be compared to a reference value stored in memory. The measured signal may be compared to a signal in absence of a string by comparing the ratio of the measurements to a reference threshold. The technique may further include determining a baseline signal value by measuring a signal in the absence of a string, storing the baseline signal value in a memory, determining a foreign object signal value by measuring a signal when a foreign object is detected, obtaining a difference value by subtracting the foreign object signal value and the baseline value from each other, and comparing the difference value to a reference value stored in the memory. In addition, the method may include determining that a substantially transparent string has been detected if the difference value is positive, and detecting that a substantially opaque string has been detected if the difference value is negative. The reference value may be defined by statistical measurement of a plurality of measurements, in the presence or in the absence of the string, computing a mean value and a standard deviation, and defining a reference value substantially equal to the mean value + or xe2x88x92 n standard deviations, and n may be between 0 and 5.
The details of various embodiments of the invention are set forth in the accompanying drawings and the detailed description. Other features and advantages of the invention will be apparent from the description and drawings, and from the claims.